Generally, an impurity semiconductor device or a pn-junction semiconductor device exhibits electric resistance exponentially decreased with increasing temperature, as shown in FIG. 1. A semiconductor device may undergo Joule heating when an electric current flows. This Joule heating reduces electric resistance of the semiconductor device, thereby increasing the electric current passing through the semiconductor device. This results in generating more heat from the semiconductor device. Occasionally, Joule heat generated by this heating mechanism may be large enough to cause serious damage on the semiconductor device. This phenomenon is called “Thermal Runaway”. The thermal runaway has caused difficulty in connecting the semiconductor devices in parallel.